Method for treating reactive metals in a vacuum furnace

ABSTRACT

The present invention is directed to a method for reducing the contamination of reactive metal melts in vacuum furnaces due to the presence of residual gaseous contaminants in the furnace atmosphere. This reduction is achieved by injecting a stream of inert gas directly over the metal confined in a substantially closed crucible with the flow of the gas being sufficient to establish a pressure differential between the interior of the crucible and the furnace atmosphere.

United States Patent 11 1 Hulsey Oct. 28, 1975 [54] METHOD FOR TREATING REACTIVE 3.061%,828 11/1962 Booth et a1. 75/96 x METALS IN A VACUUM FURNACE 5:322 gf lg 322g: 1 g:

1 1 In 1 Inventor: William J- Hulsey, Oak Ridge, 3,287,109 11/1966 11716616111161" 75/96 x Tenn. 3,467,167 9/1969 Mahin 1 1 v 75/96 X 3,470,936 10/1969 Jandras 266/34 V X [73] Asslgnee' i i sfizg i zz 3,689,048 9/1972 FOUlard et al. 75/96 x g p r gg Primary Examiner-Allen B. Curtis 3 a mg Assistant Examiner-Thomas A. Waltz [22] Filed: Jan. 8, 1974 Attorney, Agent, or Firm-John A. Horan; David S. 1 pp No 431 819 Zachry; Earl L. Larcher [5 7] ABSTRACT 52 1 U S Cl 75/84 1 2 5??? The present invention is directed to a method for re- [511 Int Cl 2 C22B 60/04 C22B 60/02 ducing the contamination of reactive metal melts in C22B 34/00 vacuum furnaces due to the presence of residual gase- [58] Field of Search 75/96 84 84 1 ous contaminants in the furnace atmosphere. This re- 5 duction is achieved by injecting a stream of inert gas directly over the metal confined in a substantially [56] References Cited closed crucible with the flow of the gas being suffi- I N cient to establish a pressure differential between the UN TED STATES PATE TS interior of the crucible and the furnace atmosphere. 2,054,921 9/1936 Betterton 266/34 V 2,054,922 9/1936 13611611611 et 111.. 266/34 v 3 Claims, N0 Drawings 2,684,297 7/1954 Urban 75/96 X METHOD FOR TREATING REACTIVE METALS IN A VACUUM FURNACE The present invention relates generally to the treatment of metallic material in a vacuum furnace, and more particularly to a method for significantly reducing the contamination of reactive metal melts in such furnaces due to the presence of gaseous contaminants in the furnace atmosphere. This invention was made in the course of, or under, a contract with the U.S. Atomic Energy Commission.

High temperature vacuum furnaces are frequently employed for heat treating, melting, or alloying various metallic materials as well as performing other metallurgical functions. The presence of residual gaseous reactants in the furnace atmosphere, under vacuum, has presented a significant drawback to the use of such furnaces when treating metals which are relatively highly susceptible to reactions with gaseous reactants such'as oxygen, hydrogen, nitrogen, etc.,. especially when the metals are in molten states. Examples of reactive metals which are relatively easily susceptible to reactions with one or more of the aforementioned gases which form the residual gases in vacuum furnaces include the actinides such as uranium, thorium and plutonium, as well as refractory type metals such as tungsten, niobium, tantalum, molybdenum, titanium, zirconium, and alloys of these metals. 7

Oxidation of metal is perhaps one of the most prevalent problems encountered while practicing metallurgical procedures in vacuum furnaces. For example, uranium metal chips, e.g., machine tumings, may be melted in a vacuum furnace for the purpose of forming a uranium ingot. However, when employing a vacuum furnace capable of being evacuated to a fairly low pressure of 150 microns, the residual oxygen remaining in the furnace atmosphere is sufficient to oxidize about to 10 percent of the uranium charge. This amount of oxidation substantially detracts fromthe practice of direct cycling of uranium scrap in forms having relatively high surface area such as chips due to low metal recovery. Recovery of the oxidized uranium employing a uranium tetrafluoride bomb reduction procedure, or the like, represents a considerable expenditure. Similarly, the purity of other reactive metals and alloys is significantly affected by the presence of residual gaseous reactants in the furnaces even after purging the furnaces several times with an inert gas of high purity.

Accordingly, it is the primary aim or objective of the present invention to provide a method for controlling the atmosphere in a vacuum furnace at locations contiguous to the metal being treated therein for the pur pose of inhibiting or substantially minimizing reactions between this metal and the residual gases within the furnace. The method of the present invention is practiced by the steps of confining a reactive metal or alloy in a substantially closed crucible within the furnace, evacuating the fumace to the desired partial pressure, and then injecting a stream of inert gas into the crucible directly over the metal during the high temperature operation. This stream of inert gas is injected at a flow rate sufficient to provide a pressure higher than the pressure within the furnace so as to form a pressure differential between the interior of the crucible and the furnace atmosphere for shielding exposed surfaces of the reactive metal from contaminants in the furnace atmosphere.

Other and further objects of the invention will be obvious upon an understanding of the illustrative method about to be described, or will be indicated in the appended claims, and various advantages not referred to herein will occur to one.skilled in the art upon employment of the invention in practice.

Metallurgical furnaces capable of operating under vacuum at pressures as low as about microns have not been found to be sufficiently efficient to reduce the residual oxygen content in the furnace environment to levels for preventing substantial. oxidation of reactive metals being treated in the furnace. For example, in such furnaces about 5 percent or more of the metal may be oxidized if the reactive metal is treated in a form having high surface area, such as chips and scraps. In the method of the present invention the reactive metal or alloy is placed in a cruciible in the furnace. The crucible is of a suitable well-known refractory material, e.g., zirconia coated graphite, and is provided with a lid or cover disposed thereon in a non-airtight fit. After evacuating the furnace by employing conventional pumping equipment a stream of an inert gas such as argon or helium is injected through a suitable conduit into the crucible at a location over the metal confined therein. With the pressure inside the furnace being at a lower pressure than the pressure within the crucible due to the presence of the inert gas within the crucible a low pressure area is thus provided for receiving the inert gas escaping from the crucible. This pressure differential is provided by using a relatively low flow rate of inert gas for assuring that the impurities in the inert gas will not be sufficient to cause undesirable contamination of the metal charge and yet be sufficient to dilute and flush gases from the crucible and metal charge. Also, with this higher pressure within the crucible low pressure reactive gases which are present in the furnace interior are prevented from entering the crucible and contaminating the metal charge confined therein. Injections of fairly large quantities of inert gas into the furnace such as used during a purge, or the like, would not give the same results with or without the use of vacuum since the content of impurities in such large quantities of inert gases would react with the metal charge to cause significant corrosion of the latter.

The flow rate of the inert gas found to be satisfactory for practicing the method of the present invention is in the range from an effective amount sufiicient to create a finite pressure differential between the interior of the crucible and the furnace atmosphere to about 5 liters per minute. These fiow rates are satisfactory for a furnace having a capacity of about cubic feet with a closed crucible therein having a. furnace leak rate of about 5000 microns.cubic foot/minute. Of course, if a crucible having a leak rate which would inhibit forming the necessary pressure differential is used, then a greater flow rate of the inert gas would be required. Normally, an average flow rate of about 2 liters per minute of argon containing about. 10 ppm oxygen is adequate with a furnace operating at microns. At this flow rate and oxygen content only about 0.013 grams of uranium would be oxidized in a l-hour period. Accordingly, with the low flow rate of inert gas necessary to practice the invention the purity of the gas is not particularly critical so as to allow for the usage of inert gas having a purity in the range of that obtainable in commercially available gases, i.e., about 99.9 percent pure. Employing a flow rate of about 2 liters of argon (containingabout 10 ppm oxygen) per minute with a partial pressure of about 200 microns of argon in a furnace operating at l50.microns pressure the partial pressure of the oxygen in the crucible would be a very low 2 X 1O torr, which pressure would be extremely difficult to obtain, even in highly sophisticated and expensive vacuum furnaces.

In a typical operation of the present invention a length of graphite tubing of a 0.625-inch diameter which is capable of easily conveying the required argon flow of about 2 liters per minute was passed through the wall of the vacuum casting furnace and through a suitable aperture in the crucible -lid. The tubing extended into the crucible near the center thereof to assure that the flowing argon covered the'entire charge of metal in the crucible. With a charge of uranium metal chips weighing 380 kg in the crucible and the furnace evacuatedto 150 microns pressure, argon having about 10 ppm oxygen was injected at a flow rate of 2 liters per minute into the crucible through the graphite tubing while the furnace temperature was increased to 1400C. over a 35-hour duration and then reduced to l350C. and held for 0.5 hour to effect the melting. in this operation a total of only 1500 grams of the uranium charge was oxidized. Comparatively speaking, with a similar charge of uranium chips confined in a vacuum furnaceevacuated to the same pressure level and subjected to the same heating operations a total of 18,000 to 38,000 grams of the uranium metal would be oxidized during the melting operation due to reactions with residual gases in the furnace atmosphere. This represents a total of 5 to percent of the uranium charge.

It will be seen that the present invention affords a relatively simple technique for substantially eliminating corrosion of metal treated in vacuum furnaces due to the presence of residual gaseous reactants. Alternatively, the subject process may be employed for controlling gasmetal reactions in which gas product removal or low oxygen pressures are desirable. For example, carbon may be removed from uranium or other metals by reacting dissolved carbon with oxygen to produce carbon monoxide which can be removed to a very low partial pressure by the low pressure inert gas'injection system of the present invention.

I claim:

1. A method for inhibiting the contamination ofa reactive metal during a melting operation at a temperature at which such contamination occurs in a vacuum furnace due to contact between residual gaseous reactants in the furnace atmosphere and said reactive metal, comprising the steps of placing a crucible con? taining a charge of the reactive metal in the vacuum furnace with said crucible having a lid thereon, evacuating the furnace to provide an atmosphere therein at a pressure less than atmospheric pressure, injecting a stream of gas inert to said metal first into said crucible directly over said metal and then from said crucible into said furnace atmosphere during said melting operation to prevent reaction of said metal with said residual gaseous reactants, said crucible having an aperture therethrough to provide a passageway for conveying said inert gas from withinsaid crucible into the furnace atmosphere, said inert gas being injected into said crucible at a flow rate sufficient to provide a pressure higher than the pressure in the furnace atmosphere so as to form and maintain a pressure differential between the atmosphere in the crucible and the atmosphere in the furnace for preventing reaction of the reactive metal in said crucible with said residual gases in the furnace while simultaneously diluting and flushing gases from within the crucible and the reactive metal via said passageway.

2. The method claimed in claim 1, wherein the reactive metal is a metal selected from the group consisting of uranium, plutonium, thorium, tungsten, tantalum,

niobium, molybdenum, titanium, zirconium, and alloys thereof, and wherein the inert gas is selected from the group consisting of argon and helium.

3. The method claimed in claim 2, wherein the flow rate of inert gas injected into the crucible is in the range defined by a flow rate sufficient to provide said higher pressure to about 5 litersper minute. 

1. A METHOD FOR INHIBITING THE CONTAMINATION OF A REACTIVE METAL DURING A MELTING OPERATION AT A TEMPERATURE AT WHICH SUCH CONTAMINATION OCCURS IN A VACUUM FURNACE DUE TO CONTACT BETWEEN RESIDUAL GASEOUS REACTANTS IN THE FURNACE ATMOSPHERE AND SAID REACTIVE METAL, COAMPRISING HE STEPS OF PLACING A CRUCIBLE CONTAINING A CHARGE OF THE REACTIVE METAL IN THE VACUUM FURNACE WITH SAID CRUCIBLE HAVING A LID THEREIN, EVACUATING THE FURNACE TO PROVIDE AN ATMOSPHERE THEREIN AT A PRESSURE LESS THAN ATMOSPHERIC PRESSURE, INJECTING A STREAM OF GAS INERT TO SAID METAL FIRST INTO SAID CRUCIBLE DIRECTLY OVER SAID METAL AND THEN FROM SAID CRUCIBLE INTO SAID FURNACE ATMOSPHERE DURING SAID MELTING OPEATION TO PREVENT REACTION OF SAID METAL WITH SAID RESIDUAL GASEOUS REACTANTS, SAID CRUCIBLE HAVING AN APERTURE THERETHROUGH TO PROVIDE A PASSAGEWAY FOR CONVEYING SAID INERT GAS FROM WITHIN SAID CRUCIBLE INTO THE FURNACE ATMOSPHERE, SAID INERT GAS BEING INJECTED INTO SAID CRUCIBLE AT A FLOW RATE SUFFICIENT TO PROVIDE A PRESSURE HIGHER THAN TE PRESSURE IN THE FURNACE ATMOSPHERE IN THE MAINTAIN A PRESSURE DIFFERENTIAL BETWEEN THE ATMOSPHERE IN THE CRUCIBLE AND THE ATMOSPHERE IN THE FURNACE FOR PREVENTING REACTION OF THE REACTIVE METAL IN SAID CRUCIBLE WITH SAID RESIDUAL GASES IN THE FURNACE WHILE SIMULTANEOUSLY DILUTING AND FLUSHING GASES FROM WITHIN THE CRUCIBLE AND THE REACTIVE METAL VIA SAID PASSAGEWAY.
 2. The method claimed in claim 1, wherein the reactive metal is a metal selected from the group consisting of uranium, plutonium, thorium, tungsten, tantalum, niobium, molybdenum, titanium, zirconium, and alloys thereof, and wherein the inert gas is selected from the group consisting of argon and helium.
 3. The method claimed in claim 2, wherein the flow rate of inert gas injected into the crucible is in the range defined by a flow rate sufficient to provide said higher pressure to about 5 liters per minute. 